HLSL: workaround FXC sometimes failing with 'not all control paths return a result' in functions with discard

To fix this, we trick the compiler by wrapping the function body with an
if (true) { <function body> } followed by returning an unused value of
the return type.

Bug: tint:1081
Change-Id: I763bf768f40d07a1045f0a70017bb40d488c8428
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/68822
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

src/resolver/resolver.cc

@@ -2166,6 +2166,7 @@ bool Resolver::Statement(const ast::Statement* stmt) {
       }
       return false;
     }
+    current_function_->SetHasDiscard();
     return true;
   }
   if (stmt->Is<ast::FallthroughStatement>()) {